Structure cristalline de la phase β-KEr₂F₇. Composés isotypes

Abstract

Potassium erbium fluoride β-KEr₂F₇ crystallizes in the orthorhombic system, space group Pna2₁, with the unit-cell dimensions a = 11.820, b = 13.333, c = 7.816Å (Z = 8). The crystal structure has been solved from single-crystal diffractometer measurements (AgKα) by Patterson and Fourier syntheses and refined by a least-squares method. The final R value is 0.042 for 2374 independent observed reflections (R_W = 0.051). The four species of erbium atoms are surrounded by eight fluorine atoms. These fluorine atoms form, respectively, three quadratic antiprisms and one dodecahedron, derived from a distorted cube. Two antiprisms and the dodecahedron share two of their faces to form (Er₃F₁₇)⁸⁻ groups. These groups are bidimensionally linked and the planes they form are joined together by the third antiprism. A three-dimensional network is then produced, in the tunnels of which potassium atoms are located. Lattice parameters of compounds which are isotypic to the new structure type of β-KEr₂F₇ are given.